by Anthony Barreiro » Wed Oct 23, 2013 3:59 pm
Nitpicker wrote:Nitpicker wrote:Possibly foolishly, I'm more inclined to think that the software is using stale orbital elements, or something like that, as these little moon orbits can get pretty chaotic.
I just found perhaps the best source of ephemerides (as opposed to the less accurate mean orbital elements) from the JPL's HORIZONS system, viz:
http://ssd.jpl.nasa.gov/horizons.cgi
Using it, I generated the following table for five of Saturn's moons, for the day this APOD was taken:
Saturn_Moon_Ephemerides.PNG
I included Dione in the table as a reference, as its orbit is more widely known than the other four small moons under discussion in this topic.
The
Ang values (a.k.a. the "SatPANG" values generated by the HORIZONS system) are angles in degrees from the North Celestial Pole (of Earth or Saturn?), measured anti-clockwise from Saturn-centre to moon-centre, with zero calibrated to about midnight and 180 calibrated to about midday on Saturn (as best I can tell).
What it shows, I believe, is that the four small moons are orbiting in the same order for the whole day, with Janus leading, followed by Epimetheus, then Pandora and Prometheus trailing. I believe this is enough to question the accuracy of
Sky Safari's small moon simulations, as described by Anthony.
But I'm still struggling to positively identify the left-most dot in this APOD. It seems to be ahead of where Janus should be. I'm beginning to think there are still are few important unknowns in this puzzle, such as the time between exposures in this mosaic, the level of image processing done, and whether the
Ang is measured from the Earth's pole or Saturn's pole. Regardless, I'm still having fun and learning a lot!
I hadn't thought about the time between the individual exposures used to make this mosaic -- that may account for the discrepancy. The positions of Pandora followed by Prometheus are consistent between the apod image, sky safari, and the jpl ephemeris. So I would guess that Pandora and Prometheus are in one image, and the other little bit (which looks more and more like a moon and less and less like a star every time I look at it) is in another image taken at another time. And given that these moons all complete full orbits in 15 to 17 hours, they could all be anywhere over the course of "10 October 2013." The intention of the apod image is to show Saturn and the rings as a whole from above Saturn's orbit, not to show the locations of tiny moons at one moment in time.
Here's another Cassini image, a single exposure of
Prometheus, Pandora, and Epimetheus, taken in 2004, just for comparison.
By the way, Sky Safari updates solar system object positions from NASA ephemerides, not just by calculation based on orbital elements. I haven't found any glitches previously, so I think the mosaic time lag is a more likely explanation for that little mystery spot.
[quote="Nitpicker"][quote="Nitpicker"]Possibly foolishly, I'm more inclined to think that the software is using stale orbital elements, or something like that, as these little moon orbits can get pretty chaotic.[/quote]
I just found perhaps the best source of ephemerides (as opposed to the less accurate mean orbital elements) from the JPL's HORIZONS system, viz:
http://ssd.jpl.nasa.gov/horizons.cgi
Using it, I generated the following table for five of Saturn's moons, for the day this APOD was taken:
[attachment=0]Saturn_Moon_Ephemerides.PNG[/attachment]
I included Dione in the table as a reference, as its orbit is more widely known than the other four small moons under discussion in this topic.
The [b]Ang [/b]values (a.k.a. the "SatPANG" values generated by the HORIZONS system) are angles in degrees from the North Celestial Pole (of Earth or Saturn?), measured anti-clockwise from Saturn-centre to moon-centre, with zero calibrated to about midnight and 180 calibrated to about midday on Saturn (as best I can tell).
What it shows, I believe, is that the four small moons are orbiting in the same order for the whole day, with Janus leading, followed by Epimetheus, then Pandora and Prometheus trailing. I believe this is enough to question the accuracy of [i]Sky Safari[/i]'s small moon simulations, as described by Anthony.
But I'm still struggling to positively identify the left-most dot in this APOD. It seems to be ahead of where Janus should be. I'm beginning to think there are still are few important unknowns in this puzzle, such as the time between exposures in this mosaic, the level of image processing done, and whether the [b]Ang [/b]is measured from the Earth's pole or Saturn's pole. Regardless, I'm still having fun and learning a lot![/quote]
I hadn't thought about the time between the individual exposures used to make this mosaic -- that may account for the discrepancy. The positions of Pandora followed by Prometheus are consistent between the apod image, sky safari, and the jpl ephemeris. So I would guess that Pandora and Prometheus are in one image, and the other little bit (which looks more and more like a moon and less and less like a star every time I look at it) is in another image taken at another time. And given that these moons all complete full orbits in 15 to 17 hours, they could all be anywhere over the course of "10 October 2013." The intention of the apod image is to show Saturn and the rings as a whole from above Saturn's orbit, not to show the locations of tiny moons at one moment in time.
Here's another Cassini image, a single exposure of [url=http://photojournal.jpl.nasa.gov/catalog/PIA05393]Prometheus, Pandora, and Epimetheus[/url], taken in 2004, just for comparison.
By the way, Sky Safari updates solar system object positions from NASA ephemerides, not just by calculation based on orbital elements. I haven't found any glitches previously, so I think the mosaic time lag is a more likely explanation for that little mystery spot.